1. Field of the Invention
This invention relates to high strength, high stiffness magnesium base metal alloy composites, and more particularly to products made from a mixture containing rapidly solidified magnesium alloy powders and SiC particulate using liquid suspension coprocessing or mechanical alloying followed by consolidation to bulk articles.
2. Description of the Prior Art
Magnesium alloys are considered attractive candidates for structural use in aerospace and automotive industries because of their light weight, high strength to weight ratio, and high specific stiffness at both room and elevated temperatures. However, their low mechanical strength, low stiffness, and poor corrosion resistance have prevented wide scale use of magnesium alloys. Furthermore, the alloys are comparatively soft and are subject to galling and seizing when engaged in rubbing friction under load.
The application of rapid solidification Processing (RSP) in magnesium alloys results in the refinement of grain size and intermetallic particle size, extended solid solubility, and improved chemical homogeneity. By selecting the thermally stable intermetallic compound (Mg.sub.2 Si) to pin the grain boundary during consolidation, a significant improvement in the mechanical strength [0.2% yield strength (TYS) up to 393 MPa (57 ksi), ultimate tensile strength (UTS) up to 448 MPa (65 ksi), elongation (El.) up to 9%] can be achieved in RSP Mg-Al-Zn-Si alloys, [S. K. Das et al., U.S. Pat. No. 4,675,157, High Strength Rapidly Solidified Magnesium Base Metal Alloys, June 1987]. Addition of rare earth elements (Y, Nd, Pr, Ce) to Mg-Al-Zn alloys further improves corrosion resistance (11 mdd when immersed in 3% NaCl aqueous solution for 3.4.times.10.sup.5 sec at 27.degree. C.) and mechanical properties [TYS up to 435 MPa (63 ksi), UTS up to 476 MPa (69 ksi), El up to 14%] of magnesium alloys, [S. K. Das & C. F. Chang, U.S. Pat. No. 4,765,954, Rapidly Solidified High Strength, Corrosion Resistant Magnesium Base Metal Alloys, August 1988].
Metal matrix composites (MMC's) have been the subject of intense research and development within the past ten years. Metal matrix composites consist of a metal base that is reinforced with one or more constituents, such as continuous graphite, alumina, silicon carbide, or boron fibers and discontinuous graphite or ceramic materials in particulate or whisker form. By combining the high strength, stiffness, and wear resistance of ceramics with the toughness and formability of metals, MMC's provide mechanical properties markedly superior to those of unreinforced alloys of comparable density. The incorporation of hard phases as reinforcements to a magnesium matrix can result in enhanced specific strength and specific modulus as compared to the monolithic materials.
There are currently two types of magnesium composites: continuous fiber reinforced and particulate/whisker reinforced magnesium. In the case of continuous fiber reinforced composites, the fiber is the dominating constituent, and the magnesium matrix serves as a vehicle for transmitting the load of reinforcing fiber. Properties of continuous fiber reinforced composites rely on the filament properties and the capability of the fiber/matrix interface to transfer load. Composites that incorporate discontinuous reinforcement are matrix dominated, forming a pseudo dispersion hardened structure. The primary strengthening mechanism is the retardation of dislocation movements by the fine dispersion of reinforcement.
Three distinct methods have been used to prepare magnesium metal-matrix composites: a liquid metal (melt) infiltration method, a semi-solid metal forming method, and a powder metallurgy (P/M) method.
Liquid metal methods for the fabrication of metal matrix composites have the advantages of relative simplicity, flexibility, economy, and ease of production of complex shapes, [A. Mortensen et al., Solidification Processing of Metal-Matrix Composites, Journal of Metals, 40, Feb. 1988, pp. 12-19], [P. Rohatgi, Cast Metal-Matrix Composites, Metals Handbook, Ninth Edition, 15, 1988, pp. 840-854]. A basic requirement of liquid metal processing of composites is the intimate contact and bonding between the reinforcement and the molten alloy. This requirement may be met either by mixing the reinforcement, generally a form of particulate, into the partially or fully molten alloy, or by the use of pressure to infiltrate reinforcement preforms with liquid metal. For those casting processes requiring ceramic preforms, the wettability of the ceramic reinforcement by the metal matrix alloy particularly affects the pressure requirements for infiltration, the quality of the interface bond and the nature of the defects in the resultant casting. For casting processes which depend upon introducing and dispersing a reinforcement into a melt or vigorously agitated, partially solidified slurries, a number of techniques have been developed. Examples include addition or injection of particles to a vigorously agitated alloy; dispersion of pellets or briquettes in a mildly agitated melt; powder addition in an ultrasonically agitated melt; addition of powders to an electromagnetically stirred melt; and centrifugal dispersion of particles in a melt, [P. Rohatgi, Foundary Processing of Metal Matrix Composites, Modern Casting, April 1988, pp. 47-50].
Semi-solid metal (SSM) forming incorporates both casting and forging [M. P. Kenney, et al., Semisolid Metal Casting and Forging", Metals Handbook, Ninth Edition, 15, 1988, pp. 327-338]. The process involves mixing of a particulate reinforcement into a molten magnesium alloy, followed by direct chill (DC) casting of the composite under conditions of magnetohydrodynamic (MHD) stirring. These steps yield a microstructure, which when reheated to the semi-solid state, responds to forming into near net shape components.
Powder metallurgy MMC's, which require considerable time and care to produce, typically have tensile and fatigue properties superior to those of melt-infiltrated composites due to the advantages of lower temperature processing which reduces the chance of interface reaction, and blending of powder/reinforcement constituents which are incompatible in liquid state handling.
The P/M process starts with mixing and blending prealloyed metallic powder and reinforcement particulates/whiskers, followed by heating and degassing, and finally consolidation into intermediate or final product forms. During the critical states of production, measured quantities of reinforcement constituents and fine mesh metal alloy powders are thoroughly mixed and blended to establish a high degree of particle intermingling. Lubricants and selected additives are usually employed in this kind of metal and ceramic multicomponent powder system to help overcome some of the problems inherent to the mechanics of mixing, [P. E. Hood and J. O. Pickens, Silicon Carbide Whisker Composites, U.S. Pat. No. 4,463,058, July 1984]. The adverse effects of interparticle friction, electrostatic attraction, and density differences must be reduced to facilitate flow during mixing and blending. Mechanically interlocked agglomerates of whiskers also must be separated to establish a statistically random dispersion. This can normally be achieved with high-velocity high-shear blending equipment. The production of dense, porosity-free MMC's by a P/M process critically depends on proper treatment of the composite powder blends to remove volatile contaminants effectively. Residual organics, such as lubricants and other mixing and blending additives, must be completely extracted before consolidation. Water vapor and gases adsorbed to the particle surfaces must also be removed.
Each of the previously P/M processes uses conventional gas atomized magnesium alloy powder in the matrix. A number of other variations exist for powder processing of composites. However, the blending, pressing and sintering steps are virtually all regarded as proprietary technology, and thus very few details are available in the published literature.
There remains a need in the art for high strength high stiffness magnesium base metal alloy composites having the form of bulk articles consolidated from a powder mixture containing rapidly solidified magnesium alloy powders and SiC particulate.